ENH: Update SlicerExtension-VMTK extension

This updates the SlicerExtension-VMTK extension to
f5b79887b2dc00b18b06e4299256916f3c188578.
Merge pull request #668 from jcfr/update-SlicerExtension-VMTK-master

ENH: Update SlicerExtension-VMTK extension